\Header{list}{Lists -- Generic and Dotted Pairs}
\alias{alist}{list}
\alias{as.list}{list}
\methalias{as.list.data.frame}{list}
\methalias{as.list.default}{list}
\alias{as.pairlist}{list}
\alias{is.list}{list}
\alias{is.pairlist}{list}
\alias{pairlist}{list}
\keyword{list}{list}
\keyword{manip}{list}
\begin{Description}\relax
Functions to construct, coerce and check for all kinds of \R{} lists.\end{Description}
\begin{Usage}
\begin{verbatim}
list(...)
pairlist(...)

as.list(x, ...)
as.pairlist(x)

is.list(x)
is.pairlist(x)

alist(...)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{...}] objects.
\item[\code{x}] object to be coerced or tested.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
Most lists in \R{} internally are \emph{Generic
Vectors}, whereas traditional \emph{dotted pair} lists (as in LISP)
are still available.

The arguments to \code{list} or \code{pairlist} are of the form
\code{value} or \code{tag=value}.
The functions return a list composed of its arguments
with each value either tagged or untagged,
depending on how the argument was specified.

\code{alist} is like \code{list}, except in the handling of tagged
arguments with no value. These are handled as if they described
function arguments with no default (cf. \code{\Link{formals}}), whereas
\code{list} simply ignores them.

\code{as.list} attempts to coerce its argument to list type.
For functions, this returns the concatenation of the list of formals
arguments and the function body. For expressions, the list of
constituent calls is returned.

\code{is.list} returns \code{TRUE} iff its argument
is a \code{list} \emph{or} a \code{pairlist} of \code{length}\eqn{> 0}{},
whereas \code{is.pairlist} only returns \code{TRUE} in the latter case.

\code{is.list} and \code{is.pairlist} are generic: you can write
methods to handle of specific classes of objects, see \link{InternalMethods}.

An empty pairlist, \code{pairlist()} is the same as \code{NULL}.  This
is different from \code{list()}.\end{Details}
\begin{References}\relax
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
\emph{The New S Language}.
Wadsworth \& Brooks/Cole.\end{References}
\begin{SeeAlso}\relax
\code{\Link{vector}(., mode="list")}, \code{\Link{c}}, for concatenation;
\code{\Link{formals}}.\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
data(cars)
# create a plotting structure
pts <- list(x=cars[,1], y=cars[,2])
plot(pts)

# Argument lists
f <- function()x
# Note the specification of a "..." argument:
formals(f) <- al <- alist(x=, y=2, ...=)
f
str(al)

str(pl <- as.pairlist(ps.options()))

## These are all TRUE:
is.list(pl) && is.pairlist(pl)
!is.null(list())
is.null(pairlist())
!is.list(NULL)
is.pairlist(pairlist())
is.null(as.pairlist(list()))
is.null(as.pairlist(NULL))
\end{ExampleCode}
\end{Examples}

